The reaction of the stabilisedylides 14–23 with an excess of NO2 in CH2Cl2 at room temperature gives different results depending on the structure of the starting ylide. The monacyl ylides 14–16 give the corresponding α-oxo nitriles 4 together with Ph3PO · HNO3 (24) which has been fully characterised for the first time. Under the same conditions, the ylide 18 gives 2,4-dinitrobenzonitrile (26), Ph3PO